358 373 ch20 re 896315.qxd 3/26/10 1:57 am page 363 s-034 ...€¦ · an alpha particle is made of...

Reading Essentials 363

When you read the word radiation, what do you think of?Brainstorm some words or phrases that come to mind andwrite them on the lines below.

Before You Read

Nuclear RadiationWhen an unstable nucleus decays, it breaks apart. As it decays, the nucleus emits particles and energy called nuclearradiation. The three types of nuclear radiation are alpha, beta(BAY tuh), and gamma radiation. Alpha and beta radiationare particles. Gamma radiation is an electromagnetic wave.

Alpha ParticlesAn alpha particle is made of two protons and two neutrons.

During alpha radiation, the decaying nucleus emits an alphaparticle. An alpha particle is the same as the nucleus of ahelium atom. An alpha particle has a charge of 2+ and anatomic mass of 4. This means it has two protons and two neutrons. An alpha particle’s symbol is the same as the symbolof a helium (He) atom.

Alpha particles have much more mass than beta or gammaradiation and a greater electric charge. Alpha particles can penetrate, or pass through, matter, but it makes them loseenergy quickly. When alpha particles penetrate matter, theyattract negatively charged electrons from the atoms that theypass. This electric force pulls electrons away from the atoms.This process causes alpha particles to lose energy and slowdown. Alpha particles are heavier and move more slowly thanthe other two types of radiation. Alpha radiation cannot penetrate material as deeply as beta and gamma radiation.

Read to Learn

chapter Radioactivity and Nuclear Reactions

section ● Nuclear Decays and Reactions

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What You’ll Learn

■ how alpha, beta, andgamma radiation aresimilar and different

■ what the half-life of aradioactive material is

■ how radioactive datingis used

Identify the Main IdeaAs you read the section, highlight the main idea in each paragraph.

●B Compare andContrast Make a Foldable likethe one shown below. As youread this section, take notes on how alpha, beta, and gammaradiation are similar and howthey are different.

Alpha

Beta

Gamma

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How can humans be harmed by alpha particles?Alpha particles are heavy and move slowly. You can think of

them as bowling balls moving in slow motion. They may notbe able to penetrate material deeply, but they can do a lot ofdamage to whatever they hit. Alpha particles can be harmful if they are released inside the human body. These particlescan damage cells. Damaged cells do not work properly andmay cause illness and disease.

How can alpha particles help you?Smoke detectors work by emitting alpha particles. The

alpha particles collide with molecules in the air. The molecules break apart and form atoms with positive and negative charges. These charged particles flow within thesmoke detector, creating an electric circuit. If smoke particlesenter the smoke detector, they break this circuit. Once the circuit is broken, the smoke detector sounds an alarm.

What is transmutation?Recall that an alpha particle is made up of two protons

and two neutrons emitted from a decaying nucleus. Thenucleus now has two fewer protons and two fewer neutronsthan it had originally. When an atom loses protons, itbecomes a different element. Transmutation is the process ofchanging one element to a different element by the decayingprocess.

The nucleus that emitted the alpha particle has changed.It now has two fewer protons. Its atomic number is two lessthan the atomic number of the original element. The new element also has two fewer neutrons. Its mass number is four less than the original element.

The figure shows a polonium nucleus. Polonium has 84 protons and a total of 210 protons and neutrons. During transmutation, the nucleus gives off an alpha particle. The nucleusnow has 82 protons and a total of 206 protons and neutrons. Theelement with an atomic number of 82 and a mass number of 206is lead. The polonium atom has become a lead atom.

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21084Po 206

82Pb42He

Transmutation

364 CHAPTER 20 Radioactivity and Nuclear Reactions

2. Identify What is the symbol for lead?

Picture This

1. Compare Alpha particlescan be helpful and harmfulto humans. List one waythey can help and one waythey can harm.

Help:

Harm:

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Beta ParticlesA second type of radioactive decay is beta radiation.

Sometimes in an unstable nucleus, a neutron decays into aproton and emits an electron. The electron leaves the nucleus.This electron is a beta particle. A beta particle is an electronthat a neutron emits when it decays into a proton. The symbol for a beta particle is �1

0e.

How does transmutation relate to beta particles?An atom that loses a beta particle undergoes transmutation.

It changes into a different element. The figure below shows aniodine nucleus giving off a beta particle. Before the nucleusdecays, it has 53 protons and a total of 131 protons and neutrons. One of the neutrons becomes a proton by giving off a beta particle. Now, the number of protons increases byone and the number of neutrons decreases by one. The newelement, xenon, has 54 protons. Notice that the mass numberhas not changed. There is still a total of 131 protons and neutrons in the nucleus.

How can beta particles harm you?Beta particles move much faster than alpha particles

because they are smaller and lighter. Beta particles can alsopenetrate deeper into the material they hit. A beta particle canpass through a sheet of paper. A sheet of aluminum foil willstop a beta particle. Beta particles, like alpha particles, candamage cells if they are released inside the human body.

Gamma RaysThe third type of radiation is gamma radiation. Gamma

radiation is not emitted as particles, like alpha and beta radiation. Gamma radiation is emitted as electromagneticwaves called gamma rays. Gamma rays are electromagneticwaves with the highest frequencies and the shortest wavelengths in the electromagnetic spectrum. The symbol fora gamma ray is �, which is the Greek letter gamma.

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13153I 0

�1e 13154 Xe�

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3. Classify Which of the following is emitted when a neutron decays into aproton? Circle your answer.

a. alpha particle

b. beta particle

c. gamma rays

4. Identify In the figure, circle the mass numbersand underline the atomicnumbers.

Picture This

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What are the properties of gamma rays?Gamma rays have no mass and no charge. They travel at the

speed of light. A nucleus usually emits gamma rays when analpha or beta particle is created. Gamma rays can pass easilythrough a sheet of paper or aluminum foil because they travelvery fast and have no mass. It takes thick blocks of a materialsuch as concrete or lead to stop gamma rays. Gamma rayscause less damage to cells inside the human body than alphaand beta particles.

Nuclear FissionIn the 1930s, the physicist Enrico Fermi tried bombarding

uranium (U) nuclei with rapidly moving neutrons. He incorrectly thought the neutrons would combine with the nucleiand form larger, heavier nuclei. In 1938, Otto Hahn and FritzStrassmann found that when a neutron hits a uranium-235 (U-235) nucleus, the nucleus splits apart into smaller nuclei.

In 1939, Lise Meitner theorized that when a neutron hits auranium-235 nucleus, the nucleus becomes so unstable that itsplits into two smaller nuclei. The process of splitting a nucleusinto smaller nuclei is called nuclear fission. The word fissionmeans “to divide.”

What nuclei can split during nuclear fission?Only large nuclei, such as the nuclei of uranium and

plutonium, can split apart during nuclear fission.Nuclear fission begins with a neutron hitting a U-235 nucleus.

This produces a nucleus of U-236. U-236 is so unstable that itimmediately splits into a barium nucleus and a krypton nucleus.The process also produces several neutrons. The total mass ofthe products of a fission reaction is a little less than the totalmass of the original nucleus and the neutron.This small amountof missing mass becomes another product of nuclear fission.The missing mass is changed into a large amount of energy. Inthe figure on the next page, the wavy lines stand for the energyproduced through fission.

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5. Identify the productsof nuclear fission ofuranium 235.


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How are mass and energy related?The diagram above shows that in the process of nuclear

fission, some mass is lost. This mass is changed into energy.Albert Einstein proposed that mass and energy are related.He said that mass can be changed into energy and energy can be changed into mass. This is Einstein’s special theory ofrelativity. The theory says that energy in joules is equal tomass in kilograms multiplied by the speed of light squared.The mass-energy equation expresses this theory.

Energy (joules) � mass (kg) � [speed of light (m/s)]2

E � mc 2

Einstein’s theory of relativity states small amount of masscan be changed into a huge amount of energy. For example, ifone gram of mass is changed into energy, it releases about 100trillion joules of energy.

How does a chain reaction occur during nuclear fission?

In a nuclear fission reaction, neutrons are released. Theseneutrons can hit other nuclei. Then the nuclei split and emitmore neutrons. These neutrons hit other nuclei, and the fission reaction repeats. A chain reaction is a series of fissionreactions caused by neutrons released in each reaction.

If a chain reaction is uncontrolled, a huge amount of energy isreleased in a short amount of time. However, chain reactions canbe controlled. Adding materials that absorb the free neutrons isone way to control chain reactions. These materials keep theneutrons from hitting other nuclei and continuing the reaction.

Not all fission reactions cause a chain reaction. A chainreaction cannot occur if there is less than critical mass to provide enough free neutrons. The critical mass is the amountof starting material that makes each fission reaction produceat least one more fission reaction.

(Unstable nucleus)

Nuclear Fission Reaction of U-235

�n

n

n

n23592 U

9136 Kr

14256Ba

23692 U

energy

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7. Illustrate Draw a diagram of a chain reactionin the space below.

6. Calculate What is the sumof the mass numbers for Krand Ba? Why does this notequal the mass number ofU-236?

Picture This


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Nuclear FusionYou have learned that nuclear fission releases great

amounts of energy. When one nucleus of uranium-235 splits,it releases about 30 million times more energy than when onemolecule of dynamite explodes. Another type of nuclear reac-tion, called nuclear fusion, releases even more energy thannuclear fission. In a nuclear fusion reaction, two small, lightnuclei combine to form one large, heavy nucleus. Fusioncombines atomic nuclei, and fission splits nuclei apart.

How are temperature and fusion related?In a fusion reaction, two nuclei combine into one. How can

two nuclei get close enough to combine? The nuclei have to bemoving very fast. Nuclei are positively charged. Positivelycharged objects repel each other. If nuclei move fast enough,their kinetic energy overcomes the electric force pushing themapart. Then the two nuclei get close enough to combine.

A particle’s kinetic energy increases as temperature increases.The temperature must be millions of degrees Celsius for nucleito move fast enough to combine in a fusion reaction. Thesehigh temperatures exist on the Sun and other stars.

How is the Sun’s energy produced?The Sun is made mostly of hydrogen. It produces energy by

the fusion of hydrogen nuclei. The figure shows one step ofmany in the fusion process. A proton (H-1) and a hydrogenisotope (H-2) combine to form an isotope of helium (He-3). Inthe final step, two He-3 atoms fuse to form one He-4 atom andtwo protons. In this process, the extra mass changes into a hugeamount of energy. The thermal energy and light Earth receivesfrom the Sun comes from this process.

About one percent of the Sun’s mass has so far been changedinto energy. It is estimated that the Sun has enough hydrogen forfusion reactions to continue for another 5 billion years.

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9. Label the proton, thehydrogen isotope, and the helium isotope in thefigure.

Picture This

H-1

He-3

H-2

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8. Explain how two nuclei canovercome the electronicforce pushing them apart.


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1. Review the terms and definitions in the Mini Glossary above. Choose two of the termsthat are related and write a sentence using both terms.

2. Complete the table below to organize what you learned about different types of nuclearradiation. Some of the table is filled in to help you start.

alpha particle: particle made of two protons and two neutrons

that are emitted from a decaying atomic nucleus

beta particle: an electron that a neutron emits when it decays

into a proton

chain reaction: series of repeated fission reactions caused by

neutron releases from each reaction

transmutation: the process of changing one element to a

different element by the decaying process

After You ReadMini Glossary

Alpha Beta Gamma

Radiation Radiation Radiation

Symbol �

How it forms given off by the

decaying nucleus

What it’s an electron

made of

Speed it travels moves at the

speed of light

Mass has no mass

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End of

Section


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358 373 ch20 re 896315.qxd 3/26/10 1:57 am page 363 s-034 ...€¦ · an alpha particle is made of...

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